The present invention is in the general field of processing metal scrap, especially aluminum metal scrap so as to recover the metal in such scrap. The invention is particularly directed to a method for recovering metal from aluminum thermal barrier scrap.
With the ever increasing costs in energy, it becomes more and more desirable to process metal scrap, especially aluminum scrap so as to recover the aluminum so that the metal can be reutilized. Scrap aluminum is usually contaminated with two different types of contaminants, organic contaminants and inorganic contaminants. Organic contaminants most commonly consists of remnants of various types of oils, remnants of various types of coatings or paints and the like. The inorganic contaminants present may include dust particles, pigments indicated previously, minor amounts of various scrap metals other than the principal metal within the scrap and the like. Aluminum scrap will also normally contain varying amounts of aluminum oxide resulting from the oxidation of the aluminum scrap, and or from appropriate anodizing procedures employed in processing the metal from which the scrap originated.
All of these contaminants are preferably removed to as great a degree as possible from the metal scrap prior to the scrap being melted down in an appropriate furnace or melter in order to avoid interference with the operation of the furnace or melter and in order to minimize to as great a degree as reasonably possible the chances of the molten metal obtained from the furnace or melter being contaminated. It has been recognized that an effective manner of getting rid of the organic contaminants present on a scrap metal such as scrap aluminum is to heat the scrap aluminum to sufficient temperatures that substantially all of the organic contaminants will decompose and so that the resulting decomposition products will substantially all vaporize.
It has also been recognized that such scrap would be heated at a temperature which is sufficiently low so as to minimize oxidation of the aluminum and at a temperature which is sufficiently low so that there is little chance of the aluminum scrap tending to agglomerate or fuse into a body which is difficult to handle or which must be broken up. It has also been recognized that the amount of time that the scrap is heated should be controlled so that the scrap is heated no longer than is reasonably necessary to decompose the organic contaminants and is not held at an elevated temperature sufficient to accomplish such decomposition for a sufficient period for agglomeration of the aluminum particles to take place. Although a wide variety of different separation techniques based upon differences in various physical properties have been capable of being used to recover inorganic contaminants from metal scrap such as aluminum scrap, it normally has not been economic to utilize such procedures. In the recovery of aluminum from aluminum scrap various organic contaminants of an oxide character have normally been separated in a furnace or melter. The majority of such contaminants will float to the top of the bath of molten aluminum or form slag or slag-like skin of inorganic contaminants or slag-like skin of inorganic contaminants on the molten metal which can be skimmed off of the metal in accordance with well established techniques. Excessive slag formation during recovery of metal such as aluminum is disadvantageous because of increased labor costs in removing such slag and loss of metal during such removal procedures.
Various procedures have been proposed for the removal of organic contaminants from scrap such as aluminum scrap. One method has involved heating a bed or body of scrap either directly or indirectly to a point where various organic contaminants vaporize so that they can be ignited. Other procedures have involved conveying a bed of aluminum through a heated chamber while either hot gas is circulated through the bed or while a flame is directed toward the moving bed of scrap. Such procedures are disadvantageous for a variety of reasons. Whenever a flame is directed at a bed of aluminum scrap, there is a significant probability of at least some of the aluminum being oxidized. Of course, any such oxidation results in lowering of the amount of aluminum metal recovered. Further, the heating of aluminum scrap is relatively difficult to control because the quantity of organic contamination of such scrap may vary significantly. Some procedures are unsafe because of explosion hazards caused by the presence of significant quantities of organic material in the air.
In a more recent process, aluminum is recovered from aluminum scrap using a process in which aluminum scrap is fed into the upper inlet of a rotary kiln located so that the discharge end of the kiln discharges the scrap directly into a melting furnace. In such process, the kiln and the furnace are connected by appropriate conduits or ducting containing a burner and a blower so that there is a continuous gas flow through the furnace and then through the kiln. Such flow is counter-current to the direction of scrap flow in the kiln. The burner serves to maintain the temperature of the recycled gas to a designated value. With such type of systems, some of the recycled gas is bled off from the system through a vent in the furnace so it can discharge to the atmosphere. This separated gas may then pass through a recuperator so as to preheat either the air supplied to the burner to sustain combustion or the fuel burned in the burner or both. Although this process has some advantages over earlier procedures, it is considered to be disadvantageous for several reasons. The rotary kiln used with this procedure is essentially operated in a conventional manner so as to heat the scrap passing through it by the counter-current flowing gas stream. Such is not considered to tend to effect any significant removal of inorganic contaminants in the scrap and, further, if there is any removal of inorganic contaminants from the scrap, it is considered that such procedure will only convey the inorganic contaminants back to the furnace where they will tend to settle out and form slag or skin on the molten metal within the furnace. Additionally, because of the counter-current flow within the rotary kiln and the relationship of the kiln to the furnace, the temperature of the scrap discharged to the furnace will be related to the temperature within the furnace. As a consequence of this, it is considered that it is impossible to obtain the degree of temperature control in the furnace necessary for most effective removal of organic material from the scrap.
More recently, U.S. Pat. No. 4,264,060 discloses an apparatus and method for recovering scrap metal. The apparatus consists primarily of a direct-fired rotary kiln. In such a process, scrap is passed through the upper inlet of a rotary kiln while rotating the kiln so that the scrap is cascaded through the interior of the kiln as it is simultaneously moved through the kiln to the lower discharge end of the kiln, concurrently passing a gas stream at a temperature sufficient to cause decomposition and vaporization of organic material in the scrap through the kiln from the upper end to the lower end of the kiln so the gas stream contacts the scrap as it is cascaded through the interior of the kiln, and then separating the gas stream from the scrap at the discharge end of the kiln as the scrap is discharged from the kiln. The heated scrap is then directly passed to the melting furnace under such conditions that the gas stream from the kiln is isolated from the atmosphere above the melting furnace. The gas stream from the kiln is preferably passed through a dust collector to remove entrained inorganic materials and then is burned in an incinerator used to supply the hot gas stream introduced in to the inlet end of the kiln. The incinerator may also be used to burn or incinerate gas from the furnace or melter.
U.S. Pat. No. 4,411,695 discloses an improvement of the foregoing process wherein metallic scrap is decontaminated in a container by concurrently passing in the same direction both the scrap and a hot gas through the container transferring heat from the hot gas to the scrap.
Aluminum scrap recovery system wherein aluminum is recovered from scrap by volatilization of organics from the scrap in a hearth are described in U.S. Pat. No. 3,650,830 and U.S. Pat. No. 4,010,935.
U.S. Pat. No. 4,394,166 describes an aluminum scrap recycling process utilizing an indirect fired rotary kiln and an afterburner.
Although some degree of success has been achieved with the foregoing processes, especially the latter, such procedures are not as effective with aluminum scrap containing thermal barrier material. Such scrap is particularly unsuitable for direct charging to melt furnaces because of dense smoke pollution which is emitted. Chopping of the thermal barrier containing scrap into very small particles, less than one-half inch and separation of the aluminum therefrom by mechanical means, such as vibration, is not only expensive but results in excessive melt-loss.
In some thermal break shapes, the plastic or thermal barrier material is substantially bonded to the metal. Removal of the plastic by mechanical means is extremely difficult, if not impossible. In other thermal break shapes, the thermal barrier material is mechanically joined to the metal. Mechanical separation is difficult and costly even with these types of scrap shapes.